Field of the Invention
Aspects in accordance with the present invention relate to protective coated objects and method of coating objects.
Background of the Technology
Cultural heritage objects, in particular silver artifacts, represent a significant component of the world's cultural heritage, retained in its museums for current and future generations. The beauty of silver, which has attracted artists and artisans through the ages, is threatened by silver's intrinsic chemistry, particularly formation of Ag2S from trace H2S in the atmosphere. A standard protective treatment, mechanical polishing and application of nitrocellulose lacquer, requires extensive manual labor to replace every 10-20 years, is difficult to apply uniformly, and damages the surface by removing small amounts of silver, leading to irreversible loss of important surface detail.
To reduce the tarnish rate, and thus the number of times a silver object must be polished, art conservators often apply a polymeric nitrocellulose lacquer film. The nitrocellulose coating is applied manually in a time- and labor-intensive method that can take 3-40 hours per object depending on the size and complexity. Manual application methods, such as brushing, dipping or spraying, create thinner areas or holes, even in films applied by a practiced art conservator. The inherent non-uniformity limits the effective coating lifetime, as thinner areas tarnish more quickly and produces a more visible coating. Additionally, cross-linking of the polymeric nitrocellulose causes the films to appear yellow, an effect that occurs within 1-2 years after the initial application. In a museum environment, nitrocellulose coatings have been documented to last 10-20 years before objects need to be polished and coated again. Given the time intensive process of applying the nitrocellulose coatings, museums with large silver collections (>5,000 objects) are in a never-ending cycle of coating and recoating silver objects.
The use of a single layer deposited by atomic layer deposition to produce protective coatings for silver and silver alloys is also known. For example, U.S. Pat. No. 8,883,258 to Makela et al. describes a single layer coating for silver. However, single layer techniques are not adequate because during the atomic layer deposition [ALD] process, defects occur in the film where poor-quality or no metal oxide is deposited (known in the art as “pinholes”). Though the number and size of pinholes in ALD films is low compared to other thin film coating methods, even a small number pinholes present a large problem because a small number of pinholes in a single layer can drastically reduce the effective lifetime of a barrier coating. Pinholes act as short circuits for diffusion allowing corrosive species to quickly reach the metal surface. Pinholes occurring in single layer films are unimpeded and can propagate through the film thickness, shortening the lifetime of the barrier film.
Therefore, there is a need in the art for an improved coating and coating method to protect cultural heritage objects from tarnishing while also being transparent in the visible light spectrum, in particular objects made of pure silver and/or silver alloys.